The invention relates to a process for the preparation of p-chlorotoluene by chlorination of toluene in the presence of zeolite L and methylene chloride and/or chloroform.
Conventional chlorination of toluene in the presence of iron leads to a mixture of chlorotoluenes in which the o-isomer predominates over the p-isomer in a ratio of 2:1. This ratio can be shifted to values of about 1.1:1 to about 0.75:1 by cocatalysis with sulphur or sulphur compounds (European Patent Specification 292,824, U.S. Pat. No. 4,031,147 and U.S. Pat. No. 4,444,983). It is furthermore known that toluene can be converted into a chlorotoluene mixture having an o/p ratio of 1:2 in the presence of zeolite X, Y or L, without the co-use of solvents (European Patent Specification 231,662 and European Patent Specification 112,722). This ratio can be slightly improved again to about 1:3 by cocatalysis with some alcohols and/or some carboxylic acids (European Patent Specification 248,931 and European Patent Specification 154,236).
For the majority of industrial uses, p-chlorotoluene is the more useful product; it was therefore desirable for the preparation processes disclosed to date to be improved further in respect of p-selectivity.